The present invention relates to optics comprising the combination of a single graded-index lens and a single homogeneous plano-convex lens. More particularly, the invention relates to optics that has its numerical aperture (NA) increased by combining the two lenses in such a way that the convexo-spherical surface of the homogeneous lens is in contact with or slightly spaced from one lens surface of the graded-index lens. The optics is typically useful as an objective lens in an optical head that reads information from an optical disk or writes it into the disk.
In optical disk systems such as a compact disk (CD) apparatus and a digital video disk (DVD) apparatus, information is read from or written into the recording medium by means of an optical head that collects laser light with an objective lens to be focused at a point on the recording surface. In order to increase the density of recording on the medium, the diameter of the beam spot must be reduced. To this end, the objective lens desirably has a large NA on the image side. From the viewpoint of shorter replay and access times, it is also important that the objective lens be smaller in size and lighter in weight.
Under the circumstances, plastic aspheric lenses with NA of about 0.45 have been used as the objective lens for compact disks whereas glass molded aspheric lenses with NA of about 0.60 have been used as the objective lens for digital video disks.
In order to realize more compact recording apparatus that employ optical disks, the size of the objective lens they use must be further reduced. However, aspheric lenses are manufactured by pressing on molds and it is extremely difficult to fabricate microlenses with an outside diameter of 1 mm and less (see "Handbook of Micro-optics", ed. by the Institute of Optics of Japan, p. 6, 1995).
A practical example of the optics with an outside diameter of 1 mm of less is a rod lens having a refractive index distribution in a radial direction. The radial index distribution of the rod lens may be expressed by the following equation: EQU n(r).sup.2 =n.sub.0.sup.2 .multidot.{1 -(g.multidot.r).sup.2 +h.sub.4 (g.multidot.r).sup.4 +h.sub.6 (g.multidot.r).sup.6 +h.sub.8 (g.multidot.r).sup.8 +.sup..multidot..multidot..multidot.}
where r is the distance from the optical axis, n(r) is the refractive index at the position distant by r from the optical axis, n.sub.0 is the refractive index on the optical axis, r.sub.0 is the radius of the graded-index lens, g is an index distribution coefficient, and h.sub.4, h.sub.6 and h.sub.8 are index distribution coefficients.
Graded-index lenses of this type are typically manufactured by subjecting a rod of glass to ion exchange and even small-diameter lenses (.ltoreq.1 mm in .phi.) can be fabricated at low cost. In addition, the starting material per se has a positive refractive power, so even a rod lens of a simple shape having a plane surface on both sides can be used as an objective lens. The refractive power of the graded-index lens is expressed by n.sub.0 .multidot.g.multidot.r.sub.0 and the greater its value, the higher the NA of the objective lens that is fabricated. However, there is a limit on the difference in refractive index that can be attained by ion exchange and other fabrication methods and the practically feasible refractive power is not greater than about 0.70 (e.g., n.sub.0 .multidot.g.multidot.r.sub.0 .ltoreq.0.70).
The optics of Comparative Example 1 to be described later in this specification was solely comprised of a graded-index lens having a refractive power (n.sub.0 .multidot.g.multidot.r.sub.0) of 0.68 and its NA value was 0.624 on the image side. When one lens surface of the graded-index lens with n.sub.0 .multidot.g.multidot.r.sub.0 =0.68 was rendered spherical as in Comparative Example 2 also to be described below, the NA on the image side increased to 0.680.
However, the development of objective lens optics having an even greater value of NA is desired in order to achieve a further increase in the recording density. Aberrations are generally prone to occur in high-NA lenses, so it is also necessary to provide a certain measure for reducing the intensity of aberrations. What is more, in order to realize a compact apparatus and provide a faster access time at the same time, reduction in size and weight is also required.